Apparatus for measuring and indicating the fluid level in vessels

ABSTRACT

The invention relates an apparatus for measuring and indicating the fluid level in a vessel, such as larger LP-gas tubes for household purposes. 
     A pulse device is used for causing the vessel wall to vibrate, for example by causing a striking pin momentarily to thrust against the wall, and for scanning the oscillation frequency of the wall varying with the filling degree of the vessel. A transducer is provided which, for example, is a piezoelectric element abutting the wall. The output signal from the element is passed to a processing unit for evaluating the signal, and the result is passed to an indicator, which may be of the analog or digital type.

This invention relates to an apparatus for measuring and indicating thefluid level in vessels.

The liquid level in non-transparent vessels heretofore has beenindicated by means of stand pipes (for example at boilers) or floatingbodies (for example at oil or gasoline tanks). At other vessels orcontainers, such as LP-gas tubes or bottles for household purposes orthe like, level indication was not provided at all. In order to find outthe filling degree, one had to weigh the vessel with contents, whichobviously can be a complicated and troublesome procedure especially whenthe vessels are large and heavy.

The present invention offers a solution of the indication problemwithout any encroachment in the vessel. The invention is characterizedby a mechanically operating arrangement or a pulse device for causingthe vessel wall to vibrate, a transducer for sensing the vibrationprogress varying with the filling degree of the vessel, and a processingunit for evaluating the output signal of the transducer and transmittingthe result to an indicator.

The invention is described in greater detail in the following where itis applied to an LP-gas tube and reference is made to the accompanyingdrawings, in which

FIG. 1 is a schematic front view of a LP-gas tube for householdpurposes, equipped with a stationary apparatus according to theinvention, shown partially in block form,

FIG. 2 is a view of an apparatus designed as a separate accessory to beheld in the hand and formed as a pistol, part of its casing cut-off,

FIGS. 3a,b are typical curves of the output signal from the transducerof the apparatus at full and, respectively, empty tube,

FIGS. 4,5 and 6 are block diagrams for the electric arrangement withdigital indication by using only the frequency of the tube wall (FIG.4), a combination of the frequency of the tube wall and its vibrationamplitude (FIG. 5), and the reverberation time of the wall vibration(FIG. 6), and

FIG. 7 is the block diagram for an arrangement with analogousindication.

The LP-gas tube 1 in FIG. 1 rests on a support 2 indicated bydash-dotted lines. In the support a stationary pulse device 3 withstriking pin 4, and to the side of the device a transducer generallydesignated by 5 are carried. The transducer comprises a piezoelectriccrystal 6, which is movable in vertical direction and carries upwardly amass weight 7, which is influenced by an upward directed force, whichhere is represented by a compression spring 8. The pulse device and thetransducer are so located in the support 2, that the bottom wall 1a ofthe applied tube 1 will be located within the stroke length of thedevice and compresses the spring 8 so that an intimate contact with thecrystal 6 is ensured.

The output signal from the transducer 5 is passed to a processing unit 9and therefrom to an indicator 10.

The mode of operation substantially is as follows:

The striking pin 4 of the pulse device is released, for example bypressing a button (not shown), which releases a prestress, which acts onthe pin--and possibly is generated at the beginning of said pressing--sothat the bottom of the tube is subjected to a forceful thrust. Therebythe entire tube wall 1 is caused to perform decaying vibration. TheLP-gas amount in the tube exercises a damping effect on this vibrationwhich decreases with decreasing amount. This implies increased vibrationfrequency and decreasing amplitude.

The vibrations of the wall (bottom) are transferred by the piezoelectrictransducer 5 in the form of a correspondingly varying voltage to theprocessing unit 9, which processes the signal and emits a signal to theindicator 10 which is related to the frequency (or amplitude) andthereby to the filling degree of the tube. The indicator 10 thenindicates the LP-gas level, for example in the form of a pointerdeflection or in digital manner (as in the Figure).

The mechanical releasable prestress of the striking pin 4 can bereplaced by electro-dynamic switching-on or application of a strongvoltage pulse on a fixed piezoelectric crystal, which carries thestriking pin, or on a winding about a magnetostrictive rod having itsend close to or in contact with the tube bottom.

The transducer 5 in FIG. 1 can be replaced by a microphone positioned ata distance from the tube bottom. The microphone then collects theacoustic waves from the tube and emits a signal to the processing unitwhich is analogous with the output signal of the piezoelectric crystal.

The microphone alternative is shown applied to the embodiment shown inFIG. 2 where the observation has been utilized that the nature of theoutput signal of the transducer substantially is independent of thepoint of attack of the pulse device and transducer on the tube.

The pistol with the general designation 11 and the handle 11a shown inFIG. 2, thus, can be applied in any place of the tube wall. The pistolbore includes a first channel 12 for directing the sound (the acousticoscillations) to a microphone 13. In a second channel 14 the strikingpin is advanced which as in the case of a stapling pistol is actuated bya cock 15.

As in the afore-mentioned case, the output signal of the transducer(microphone 13) is passed to the processing unit 9, which is located inthe handle 11a, and the output of which is connected to the indicator10. The indicator in this case is shown to consist of a table comprisedin the pistol shell and provided with glow lamps 10a, which areextinguished successively for indicating in steps the falling level inthe tube. In FIGS. 1 and 2 is indicated that about 20% of the tubecontent is still remaining.

FIG. 3a shows the typical form of the output signal from the transducerat full tube, and 3b at emptied tube at the arrangement according toFIG. 1. It should be observed, however, that these curves also arerepresentative for the microphone alternative according to FIG. 2.

As certain unwarranted transients may arise just at the moment ofthrust, preferably a time delay can be inserted before the processing ofthe signal in the processing unit. The processing can be carried out indifferent ways obvious to the expert. Normally, the overtone peaksshould first be removed through a low-pass filter, so that the fallingsine waves indicated by dash-dotted lines are obtained. The Figuresillustrate clearly that the frequency is substantially higher at thecurve 3b (empty tube) than at the curve 3a (full tube), and also thatthe curve 3a falls more rapidly than the curve 3b, in response to thegreater damping.

The simplest indication should be obtained by coupling the filteredoutput signal of the transducer to a frequency meter, which is graduateddirectly in level values between limit values measured for full andempty tube. Alternatively, the output signal could be rectified andintegrated over a definite interval and be compared to a reference valuefor the same time interval.

In order to render possible measuring and indication on differentstandard types of vessels, a programmable microcomputer can be insertedfor programming the varying types and calibrating in view of tolerancesin the wall thickness etc.

The apparatus according to the invention can be used for measuring thelevel of all fluids, of liquids as well as of gas. Remote control forremote indication at arrangements of the type shown in FIG. 1 does notgive rise to problems.

Different combinations and modifications of the arrangement describedcan be imagined. In principle, the pulse device and transducer shouldconsist of one and the same piezoelectric element, which first causesthe vessel wall to vibrate by a voltage surge and thereafter acts as atransducer. The pulse device, further, could be arranged so as insteadof a single voltage pulses to emit a series of such pulses, either atintervals exceeding the maximum expected decay time (repeated indicationto the extent there is no memory for the indicator), or at intervalsfalling below the decay time, i.e. the pulse device acts as a vibrator.In this latter case the processing unit must be equipped so as todiscriminate between the oscillations overlapping each other, which arerepresented by the composed output signal of the transducer. This caseshould occur when, for example, the tube 1 in FIG. 1 is placed on avibrating support, which should replace the pulse device 3.

This opens the possibility of utilizing the invention also for levelindication in gasoline or oil tanks in motor vehicles where the startingof the engine positively gives rise to vibrations in the tank.

Although it should not be difficult for the expert to design thenecessary electronic evaluation circuits on the basis of the abovedirections, for reason of completeness four different solutions aredescribed below by way of example with reference to FIGS. 4-7.

FIG. 4 shows the simplest case, with relatively coarse indication of thefilling degree, viz. at 100%, 50%, 20% and 10%. The output signalemitted from the transducer G at thrust against the vessel in questionis amplified in a pre-amplifier F for adapting the signal level tocircuits of the type PLL (Phase Locked Loop) connected in parallel withadjustable frequency bands corresponding to the different fillingdegrees. These PLL emit output signals to respective AND-gates G1 andOR-gates G2 when the ingoing frequency lies within their adjustedfrequency band. For obtaining the greater accuracy (10%), the frequencybands for two PLL overlap each other, and both circuits must emit anoutput signal for indicating the level. Through the OR-gates G2indication on a digital display unit D is obtained.

In the case when greater accuracy is desired or required, especially atalmost empty vessel (for which a change in frequency cannot be observedany longer with safety), a coupling according to FIG. 5 can be usedwhere for the critical residual range (less than 10%) the condition isutilized that also the oscillation amplitude of the vessel wall varieswith the filling degree (see FIG. 3).

In the circuit according to FIG. 5, PLL in this case designates a sounddetector, i.e. a normal PLL which on the same chip has been completedwith a comparator, which by digital output signal indicates if aningoing signal lies within the frequency band in question. For thehigher filling degrees the same principle as in FIG. 4 is applied, i.e.with only frequency changes as criterium. The addition in relation toFIG. 4 are substantially individual pre-amplifiers F, preferably withlow-pass filter for obtaining a uniform amplitude over the frequencyband, more logic and a level detector ND scanning the oscillationamplitude for indicating filling degrees from 10% downward to 0%. Arectifier is comprised in the pre-amplifier F+L of the detector. For 5%or less both the logic for 10% and the level detector must emit anoutput signal. These two signals pass through an AND-gate G1 and thenprovide the indication 5% or less on the display unit D. The indicationof the 10% level is stopped by means of an exclusive OR-gate G2.

Due to the presence of the level indicator, the thrusts against thevessel are required each time to be carried out with constant force.This conditions does not apply to the frequency detection according toFIG. 4.

If desired, an analogous display of the indication is obtained simply bycoupling the output signals of the gates to a digital-analogousconverter with connected pointer instrument.

With reference to FIG. 6 the case is described where the indication isbased on the reverberation time in response to the filling degree. Theoscillation amplitude is damped more rapidly at a higher filling degreethan at a lower one (see FIG. 3).

The output signal emitted at thrust against the vessel from thetransducer G is passed to a pre-amplifier F, which adapts the signal tocorrect voltage level. Thereafter a band-pass filter BP filters out asuitable frequency range for the signal, which is rectified in anenvelope detector ED. The resulting direct voltage level is transferredto level detectors ND1 and ND2 where the detector ND1 with an upperlimit position g1 decides whether or not a thrust was carried out. Atthrust, the detector ND1 emits an output signal, which resets a computerR fed from a frequency generator FG for counting-up and via a one-shotMV opens a gate GR, so that an output signal from the level indicatorND2 can pass to a shifter SR. When the input signal from the envelopedetector ED due to the damping drops below a definite lower limit levelg2 for the detector DN2, the said output signal is emitted to theshifter for edge triggering the same. The shifter then shifts out thevalue counted up by the computer R to the display unit D directly or viaa converter, which re-calibrates the counted value to correspondingfilling degree percentage.

When a measurement is desired which is independent of the force of thethrust, the level detector ND2 can be completed with a peak hold circuitT, as indicated by dash-dotting. The peak can be voltage divided and beused as lower limit position g2.

FIG. 7, finally, shows the block diagram for recording n filling degreelevels directly in analogous way on the basis of frequency changes. Asat the circuit according to FIG. 5 pre-amplifiers F are here providedfor each PLL-circuit, and a level detector ND preceded by amplifiersplus rectifier F+L. Further are provided analogous gates (switches orrelays) AG, preceded by amplifier plus rectifier F+L in order to emitcorrect output signal within the associated function range.

Upon thrust, the output signal from the transducer G actuates thePLL-circuit set on the frequency band in question to open the analogousgate AG corresponding to correct filling degree. The output signal fromthis gate passes to a sample and hold circuit S & H intended to retainthe analogous value received and actuated by a one-shot MV in responseto the input signal level via the level detector circuit F+L+ND. Theresult is shown on the analogous display unit D, for example a pointerinstrument.

Many different variants of the circuits shown in FIGS. 4-7 are obviousto the expert.

I claim:
 1. An apparatus for measuring and indicating the fluid level invessels comprising a mechanically operating arrangement or a pulsedevice for initiating a vibration of the vessel wall, a transducer forsensing the oscillation progress varying both in frequency and amplitudewith the degree of filling of the vessel, and a processing unit forevaluating the output signal of the transducer and passing the result toan indicator.
 2. An apparatus as defined in claim 1, characterized inthat the pulse device comprises a strike member driven by a releasablepressure, for example a spring-prestressed pin.
 3. An apparatus asdefined in claim 1, characterized in that the pulse device is anelectric pulse-driven strike member, for example of piezoelectric,magnetostrictive or electrodynamic type.
 4. An apparatus as defined inany one of the preceding claims, characterized in that the transducercomprises an inertia-loaded piezoelectric element for application to thevessel wall.
 5. An apparatus as defined in claim 1, characterized inthat the transducer is a micropone for acoustically scanning thevibrations of the vessel wall at a distance therefrom.
 6. An apparatusas defined in claim 1, characterized in that the pulse device and thetransducer have a fixed location close to each other so as to form oneunit.
 7. An apparatus as defined in claim 6, characterized in that theunit is rigidly attached for stationary co-operation with the vesselwall, preferably the bottom wall of the vessel.
 8. An apparatus asdefined in claim 6, characterized in that the unit is attachedseparately at the end of a movable handle, for example of pistol shape,for momentary application to the vessel wall in an optional place.
 9. Anapparatus as defined in claim 1, characterized in that the pulse device,transducer, processing unit and possibly indicator are assembled to onesingle unit.
 10. Apparatus as in claim 1 wherein said transducer outputsignal includes a frequency component and wherein said processing unitevaluates the frequency component.